1. Field of the Invention
The invention generally relates to optical systems. More particularly, the invention relates to using an optical pulse stretcher to convert optical data encoded in a format to optical data encoded in another format.
2. Related Art
In various practical applications, it is desirable to change the shape of an optical pulse by “stretching” it to distribute its energy over a longer time interval than that of the original optical pulse. A conventional system for stretching a pulse is depicted in prior art FIG. 1. This conventional system is also known as a split-delay-sum pulse stretcher 100.
As shown in prior art FIG. 1, the split-delay-sum pulse stretcher 100 receives an input pulse 10. A pulse splitter 20 splits the input pulse 10 into two equal pulse portions 10A and 10B. A delay device 30 delays the pulse portion 10B with respect to the pulse portion 10A. Moreover, a pulse combiner 40 recombines the two equal pulse portions 10A and 10B, forming a stretched pulse 50 whose width (or duration) is greater than the input pulse's 10 width by the amount of the added delay from the delay device 30.
In order for the split-delay-sum pulse stretcher 100 to function properly, the pulse portions 10A and 10B must not add coherently in the pulse combiner 40. If the pulse portions 10A and 10B are baseband voltage pulses rather than optical pulses, there is no problem at the pulse combiner 40. However, if the pulse portions 10A and 10B are optical, there will be interference (destructive or constructive) between the combining pulse portions 10A and 10B if the light of the input optical pulse 10 has a coherence time greater than the delay provided by the delay device 30, or if other measures are not taken to prevent the interference. This interference can cause significant distortion of the shape of the stretched optical pulse 50. Also, in the case of coherent optical pulses, any temporal variation of the delay provided by the delay device 30 will cause temporal variation of the shape of the stretched optical pulse 50.
Moreover, optical data transmission can be impaired by jitter, which is a random or deterministic variation of the optical data stream's state transitions as compared to a reference clock. Optical data transmitters need to have low jitter, especially when they are used to measure the added jitter of tested devices or transmission systems. Methods exist for reducing transmitted jitter of electronic data streams, and for reducing transmitted jitter of RZ (retum-to-zero) encoded optical data streams. In a RZ optical data stream, an optical data bit representing a logic “1” (light on) includes a logic “0” (light off) for a portion (e.g., half) of the unit interval of the bit. However, there exist no practical methods for reducing the jitter in NRZ (non-return-to-zero) encoded optical data streams. In a NRZ optical data stream, an optical data bit representing a logic “1” (light on) remains at the logic “1” state for the entire unit interval of the bit.